This application claims priority from Japanese Patent Application No. 11(1999)-185554, filed Jun. 30, 1999, the entire disclosure of which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to a lithium secondary battery using electrolyte consisting of organic solvent containing lithium salt dissolved therein and more particularly to a lithium secondary battery using electrolyte the safety of which is ensured even if the battery is overcharged.
2. Description of the Prior Art
In recent years, a demand for a power source in the form of a small size and light weight battery has increased because of noticeable development of small size and light weight electronic appliances. Under such situations, a lithium secondary battery such as a lithium ion battery has been developed as a rechargeable small size and light weight battery of higher capacity for use in portable electronic appliances such as a small size video camera, a portable telephone, a not-book type personal computer and the like.
In such lithium secondary batteries, a carbon material capable of absorbing and desorbing lithium ion is used as a negative electrode active material, a lithium transition metal oxide compound such as LiCoO2, LiMn2O4 or the like is used as a positive electrode active material, and an organic solvent containing lithium salt dissolved therein is used as electrolyte. In the lithium secondary battery assembled with the components, lithium ions are desorbed from the positive electrode and absorbed into carbon during an initial charge of the battery so that the lithium secondary battery can be used as a rechargeable battery.
When the lithium secondary battery is overcharged, an excessive amount of lithium ions are extracted from the positive electrode and inserted into the negative electrode in accordance with an overcharged condition of the battery. This causes both the positive and negative electrodes to become thermally unstable, resulting in decomposition of the organic solvent of the electrolyte. As a result, the battery becomes over heated by abrupt exothermic reaction of the electrolyte, and the safety of the battery becomes over heated by abrupt exothermic reaction of the electrolyte, and the safety of the battery deteriorates. Particularly, such safety problems become very important when the energy density of the lithium secondary battery is increased.
To solve the safety problems, Japanese Patent Laid-open Publication No. 7-302614 proposes a method for enhancing the safety of the battery by addition of a small amount of aromatic compound to the electrolyte. In the method proposed in the Japanese Patent Laid-open Publication No. 7-302614, a carbon material is used for the negative electrode, and an aromatic compound such as anisol derivative having xcfx80 electron orbit at an reversible redox potential more noble than a positive electrode potential when fully charged in a molecular amount of less than 500) is added as an additive to the electrolyte of the battery. Such an aromatic compound is useful to prevent the battery from overcharge for protection of the battery.
Proposed also in Japanese Patent Laid-open Publication No. 9-106835 (corresponding with U.S. Pat. No. 5,879,834) is a method for enhancing the safety of the battery by addition of an additive to the electrolyte of the battery. In the method proposed in the Japanese Patent Laid-open Publication No. 9-106835, a carbon material is used for the negative electrode, and a small amount of biphenyl, 3-R-thiophene, 3-chlorothiophene, furan at the like is used as the additive to the electrolyte of the battery so that the internal resistance of the battery is increased by polymerization of the components at a higher battery voltage than a maximium operating voltage of the battery for protection of the battery in an overcharged condition.
In the method proposed in the Japanese Patent Laid-Open Publication 7-302614, the anisol derivative is effective to restrain overcharge of the battery but deteriorates the cycle characteristic and storage characteristic of the battery. The aromatic compound is decomposed by oxidation at an electric potential of about 4.5 V to generate gas therefrom and forms a polymerized substance for protection of the battery in an overcharged condition. However, in case the component of the electrolyte permits dissolution of the polymerized substance therein, the battery may not be protected in an overcharged condition. Consequently the aromatic compound such as anisol derivative having or electron orbit does not effect to restrain overcharge of the battery.
In the method proposed in the Japanese Patent Laid-open Publication No. 9-106835, the biphenyl used as the additive to the electrolyte is lower in polarity and lower in solubility. Accordingly, the additive is partly deposited during operation of the battery at a low temperature, resulting in deterioration of the battery performance. In addition, 3-chlorothiophene causes difficulty in its handling since it is stimulus and malodorous and is easily decomposed by oxidation. Similarly, the furan is also easily decomposed by oxidation. This results in deterioration of the battery performance.
To solve the problems discussed above, a primary object of the present invention is directed to provide a lithium secondary battery in which an additive effective for preventing overcharge of the battery is added to electrolyte to ensure the safety of the battery without causing any deterioration of the cycle characteristic and storage characteristics at a low temperature.
According to the present invention, the object is accomplished by providing a lithium secondary battery which includes a cell casing provided with a current interrupt device for cutting off a charge current of the battery when an internal gas pressure of the battery exceeds a predetermined value, wherein the cell casing is filled with organic solvent containing an additive such as aklylbenzene derivative or cycloalkylbenzene derivative having tertiary carbon adjacent a phenyl group. As the tertiary carbon adjacent the phenyl group is active and higher in reaction, hydrogen atom is easily extracted from the tertiary carbon in an overcharged condition of the battery. This causes rapid decomposing reaction of the additive when the battery is overcharged. As a result, hydrogen gas generates and polymer of the additives is produced by polymerization reaction of the additive. In the lithium secondary battery, when the internal gas pressure of the battery exceeds the predetermined value, the current interrupt device is operated to cut off the charge current of the battery. From the foregoing fact, it is presumed that addition of the additive to the organic solvent is effective to restrain decomposition of the electrolyte thereby to ensure the safety of the battery. In this respect, it is noted that the polymer of the additives acts as a resistance substance in an overcharged condition of the battery and does not dissolve in the electrolyte. This is useful to effectively protect the battery against overcharge.
Since the additive added to the organic solvent is in a liquid state at a room temperature, higher in polarity and higher in dissolubility to the electrolyte, the additive may not be deposited during operation of the battery at a low temperature and does not cause any deterioration of the battery performance even if added to the electrolyte. Accordingly, in use of the electrolyte added with the additive together with lithium salt dissolved in the organic solvent, the additive is useful to ensure the safety of the battery without causing any deterioration of the low temperature characteristic and storage characteristics of the battery.
In a practical embodiment of the present invention, it is desirable that the alkylbenzene derivative is at least one of additives selected from the group consisting of isopropylbenzene (cumene), 1,3-diisopropylbenzene, 1,4-diisopropylbenzene, 1-methylpropylbenzene, 1,3-bis(1-methylpropyl) benzene and 1,4-bis(1-methylpropyl) benzene. It is also desirable that the cycloalkylbenzene derivative is selected from either cyclohexylbenzene or cyclopentylbenzene.